1. Field of the Invention
This invention relates to a method for the formation of a multilayer film and more particularly to a method for the formation of a multilayer film constituting itself such a semiconductor device as a thin-film transistor.
2. Description of the Related Art
Methods for forming a multilayer film on a substrate by plasma discharge have been known for long. One of the methods utilizes parallel planar electrodes to generate plasma. This method consists in forming a deposited film on a substrate or on a film already formed on a substrate by introducing a material gas into a reduced-pressure reaction chamber provided with parallel planar electrodes and supplying a high-frequency electric power between the electrodes thereby generating a plasma therein. To generate the plasma state, a microwave discharge and a electron cyclotron resonance have been known other than high frequency discharge. A multilayer film is formed by repeating this procedure.
The manufacture of thin-film transistors, solar cells, etc. by the application of the method for the formation of a multilayer film to processes for the production of semiconductor devices has been realized. Particularly, thin-film transistors (TFT) have been attracting keen attention on account of their adaptability for liquid crystal display devices and contact sensors. As respects the TFT's for use in the liquid crystal display devices, amorphous silicon (a-Si) type semiconductors are mainly used because they allow their active layers to enjoy a large increase in surface area and they can be formed at a relatively low temperature. For the a-Si type TFT's, the inverted staggered (bottom gate) construction or the normal staggered (top gate) construction in which a gate electrode is disposed on one side and a source electrode and a drain electrode are disposed on the other side of a semiconductor film formed of an a-Si film to serve as an active layer are adopted often.
For the multilayer films which function as semiconductor devices, the qualities of the interfaces between the component films of the multilayer film are extremely important. The interface between a gate insulating film and a semiconductor film in a TFT, for example, constitutes itself a channel for the TFT and plays an extremely important role in improving the switching characteristics of the TFT. The interface between the semiconductor film and a etching-stopper film on channel constitutes itself a rear channel for the TFT and plays a very important role in improving the switching characteristics of TFT similarly to the interface between the gate insulating film and the semiconductor film.
Heretofore, in the production of a inverted staggered type TFT, for example, a gas for the formation of a gate insulating film is introduced into a reduced-pressure reaction chamber provided with parallel planar electrodes and a high-frequency electric power is supplied between the electrodes so as to generate a plasma state and deposit a gate insulating film on a glass insulating substrate having a gate electrode formed in advance thereon. Subsequently, the plasma state is stopped, the reaction chamber is evacuated, and a gas for the formation of a semiconductor is introduced into the empty reaction chamber to generate a plasma state and deposit a semiconductor film. For the purpose of further forming a etching-stopper-film on channel on the semiconductor film, the plasma state is stopped after the deposition of the semiconductor layer, the reaction chamber is evacuated, and a gas for the deposition of an etching-stopper-film on the channel is introduced into the empty reaction chamber to generate a plasma state and deposit the etching-stopper-film on channel.
In the production of the TFT of the normal staggered construction, the order in which the gate insulating film and the semiconductor film are superposed is reversed.
When the formation of a multilayer film is effected by a method which comprises depositing a first film, then stopping a plasma state, and again generating a plasma state to deposit a second film, the plasma state in the initial stage of the deposition of the second film is unstable, a defective density (dangling bond) is formed near the interface between the first and the second deposited film, and the multilayer film is not formed in a desirable form because the films are blistered or separated.
When the production of a TFT is effected by a method which comprises depositing a gate insulating film or a semiconductor film as the first deposited film, then stopping a plasma state, and again generating a plasma state to deposit a semiconductor film or a gate insulating film as the second deposited film, the plasma state in the initial stage of the deposition of the second film is unstable and a defective density (dangling bond) is formed near the interface between the semiconductor layer and the gate insulating film. This method, therefore, is at a disadvantage in forming no satisfactory interface and conferring no ideal characteristics on the produced TFT. It further has the problem that the interface between the first and the second deposited film is blistered and separated to the extent of lowering the yield of production. The fact that the plasma state to be formed again after the stop of the first plasma state takes no small time before it is stabilized impairs the productivity of this method.
The same remarks hold good for the operation of forming a semiconductor film as the first deposited film and a etching-stopper-film on channel as the second deposited film.
Incidentally, a method causes a state of plasma to last between a step of cleaning and a step of film forming in the plasma CVD process for forming a metallic or a ceramic coating on the surface of a tool, a metal die, or a machine part is disclosed under the title of "method for formation of highly adhesive thin film by plasma CVD technique" in JP-A-63-79,970. This particular invention attains the retention of the state of plasma by means of an electrode adapted exclusively therefor in addition to a discharge electrode to be used for the formation of the coating.
This invention is incapable of optimizing the state of plasma because of purpose of this invention is not focused on formation of multilayer film, and it adjusts neither the distance between the electrodes nor the internal pressure of the reaction chamber during the retention of the plasma state between the steps of depositing films. It, therefore, produces a multilayer film containing ideally formed interfaces only with difficulty.